A photodiode (PD) is a type of photodetector that is capable of generating charge carriers when exposed to light or other electromagnetic energy. An avalanche photodiode (APD) is another type of photodetector device that further to the charge carrier generation includes a multiplication of the charge carriers, i.e. an internal current gain effect, which enables a high sensitivity. Hereinafter PDs and APDs are commonly simply referred to as photodiodes when features common for both types of photodetectors are discussed.
Semiconductor photodiodes fabricated using conventional planar technology comprise a vertical pn junction, i.e. a p-type semiconductor layer on an n-type semiconductor layer, or a vertical p-i-n junction, i.e. with one or more intermediate intrinsic or low doped semiconductor layers in-between the p-type and n-type layers, in-between two electrical contacts. The avalanche photodiode performs amplification of a photocurrent generated from absorbed light by applying a reverse bias voltage to its p-i-n junction to cause an avalanche multiplication under a high electric field. It is well known that the performance of semiconductor photodiodes is limited by high dark current and noise. Typically different parts of the p-i-n or pn junction of APDs are formed of different materials in order to improve the performance of e.g. the light absorption region and the avalanche multiplication region. For example Si provides low noise characteristics but limits the detectable wavelength. However, one significant contribution to the device leakage current, i.e. the dark current, originates from a fairly high defect density in the device layers. This is predominant when device layers that are non-compatible with respect to e.g. lattice strain are combined, by epitaxial growth or by wafer bonding. Further, APDs suffer from edge breakdown due to the high electrical field. The edge breakdown can partly be avoided by using so called guard rings. However this limits the active area of the device.
The size of sensitive photodetector structures is currently limited due to a disproportionate increase in dark current with an increase in surface area. In addition, the dark current tends to increase over time due to degradation of the semiconductor material. Eventually, the device fails due to short-circuiting. This effect is particularly prominent in high internal electric field devices such as APDs since high current levels will accelerate the degradation.